III-V nitride semiconductor substrate and its production method

ABSTRACT

A self-supported III-V nitride semiconductor substrate having a substantially uniform carrier concentration distribution in a surface layer existing from a top surface to a depth of at least 10 μm is produced by growing a III-V nitride semiconductor crystal while forming a plurality of projections on a crystal growth interface at the initial or intermediate stage of crystal growth; conducting the crystal growth until recesses between the projections are buried, so that the crystal growth interface becomes flat; and continuing the crystal growth to a thickness of 10 μm or more while keeping the crystal growth interface flat.

FIELD OF THE INVENTION

The present invention relates to a III-V nitride semiconductor substratehaving a low dislocation density and a substantially uniform surfacecarrier concentration distribution, and a method for producing it.

BACKGROUND OF THE INVENTION

Because nitride semiconductor materials have sufficiently wide forbiddenbands with direct interband transition, their applications toshort-wavelength light-emitting devices have been widely investigated.In addition, because of a large saturated drift velocity of electronsand the usability of a two-dimensional carrier gas in heterojunctions,their applications to electron devices are expected.

Nitride semiconductor layers for constituting these devices are obtainedby epitaxial growth on base substrates by vapor phase growth methodssuch as a metal-organic vapor-phase epitaxy method (MOVPE), a molecularbeam epitaxy method (MBE), a hydride vapor-phase epitaxy method (HVPE),etc. However, because there are no base substrates having latticeconstants matched to those of the nitride semiconductor layers, it isdifficult to grow high-quality layers, resulting in nitridesemiconductor layers with a lot of crystal defects. Because the crystaldefects hinder devices from having improved characteristics,investigation has been vigorously carried out so far to reduce thecrystal defects in the nitride semiconductor layers.

Known as a method for producing group-III nitride crystals withrelatively few crystal defects is a method in which alow-temperature-deposited layer (buffer layer) is formed on a differentsubstrate such as sapphire, etc., and an epitaxially grown layer isformed thereon. In a crystal-growing method using alow-temperature-deposited layer, AlN or GaN is deposited on a substrateof sapphire, etc. at about 500° C., and an amorphous or partiallypolycrystalline, continuous film is formed. The film is heated to about1000° C. to cause partial evaporation or crystallization, therebyforming high-density crystal nuclei, which are used as growth nuclei toobtain a GaN film with relatively good crystallinity. Even by the methodof forming the low-temperature-deposited layer, however, the resultantsubstrates are filled with considerable numbers of crystal defects suchas threading dislocations, vacancies, etc., insufficient for obtainingpresently demanded high-performance devices.

In view of the above circumstances, investigation has been intensivelycarried out to obtain methods of using a GaN substrate as a substrate,from which a crystal grows, and forming a multi-layered semiconductorfilm, on which devices are formed, on the GaN substrate. The GaNsubstrate for crystal growth is referred to herein as a self-supportedGaN substrate. The epitaxial lateral overgrowth (ELO) technology isknown as a method for forming a self-supported GaN substrate. The ELOmethod is a technology of forming a GaN layer with few dislocations bylaterally growing the GaN layer on a base substrate in openings of amask formed thereon. JP 11-251253 A discloses the production of aself-supported GaN substrate by forming a GaN layer on a sapphiresubstrate by this ELO method and then removing the sapphire substrate byetching, etc.

A facet-initiated epitaxial lateral overgrowth (FIELO) method (A. Usui,et al., Jpn. J. Appl. Phys. Vol. 36 (1997) pp. L.899-L.902) has evolvedfrom the ELO method. The FIELO method is different from the ELO methodin forming facets in openings of a silicon oxide mask in the selectivegrowth of GaN. The facets change the propagation direction ofdislocations, thereby reducing the number of threading dislocationsreaching the top surface of the epitaxially grown layer. The growth of athick GaN layer on a base substrate of sapphire, etc. by the FIELOmethod and the removal of the base substrate thereafter can provide ahigh-quality, self-supported GaN substrate with relatively few crystaldefects.

As a method for forming a low-dislocation, self-supported GaN substrate,a method of dislocation elimination by the epi-growth withinverted-pyramidal pits (DEEP method) has been developed (K. Motoki et.al., Jpn. J. Appl. Phys. Vol. 40, JP 2003-165799 A). The DEEP methodintentionally forms a plurality of pits surrounded by facet planes on acrystal surface by growing GaN with a mask of silicon nitride, etc.patterned on a GaAs substrate, and concentrates dislocations at thebottom of the pits to lower a dislocation density in other regions.

In an as-grown state, the GaN substrate obtained by the ELO method orthe DEEP method usually has morphology such as pits, hillocks, etc. onits surface, resulting in difficulty in growing an epitaxial layer forproducing devices without further treatment. Therefore, the top surfaceof the substrate is generally mirror-polished before devices areproduced thereon.

In such circumstances, JP 2003-178984 A proposes a method for producinga group-III nitride semiconductor substrate having a low dislocationdensity, which comprises the steps of forming a metal film on a basesubstrate composed of a sapphire substrate and a first group-III nitridesemiconductor layer deposited thereon, or on a base substrate made of afirst group-III nitride semiconductor; heat-treating the base substratein an atmosphere comprising a hydrogen gas or a hydrogen-containingcompound gas to form voids in the first group-III nitride semiconductorlayer; and forming a second group-III nitride semiconductor layer on themetal film. JP 2003-178984 A describes in Example 14 and FIG. 16 aself-supported GaN substrate, whose fluorescent photomicrograph in thecross section shows no black stripes but substantially uniform blackportions near an peeling interface with the sapphire substrate. Withrespect to this phenomenon, JP 2003-178984 A describes that increase inthe amount of hydrogen in a carrier gas suppresses defects from growingto the surface.

It has been found, however, that the self-supported GaN substrateproduced by such a method has a nonuniform carrier concentration on itssurface despite the lowered dislocation density. The nonuniform carrierconcentration distribution on its substrate surface is a problem thathas never occurred in conventionally used semiconductor substrates of Siand GaAs because of their production methods. In the self-supported GaNsubstrate, however, there may be locally nonuniform regions in thecarrier concentration because an epitaxially grown thick crystal layeris used as the substrate. When crystal growth is carried out whileforming facets in a growth interface, to lower the dislocation of theself-supported GaN substrate, there inevitably occurs difference betweenfacet planes and other planes, resulting in different rates of crystalgrowth and thus differences in effective segregation coefficients ofimpurities therebetween, which leads to the nonuniform impuritydistributions, namely variations in the carrier concentrations. Becauseregions with different carrier concentrations appear as the hysteresisof facet-grown regions, they are distributed such that they extend in acrystal growth direction. If the regions with different carrierconcentrations reached the top surface of the substrate, variations inthe carrier concentration would inevitably occur on the top surface ofthe substrate.

It has been found that when there are regions having nonuniform carrierconcentrations on the surface of the GaN substrate, epitaxial GaN layersgrown on such regions are prone to have large surface roughness. Namely,even if an underlying GaN substrate is mirror-polished, there occurs aphenomenon that the resultant epitaxial layer has a rough surface.Without an epitaxial GaN layer having a uniform surface morphology, thecharacteristics of devices formed thereon would suffer fromdeterioration, variations, etc.

When a crystal grows while forming pits surrounded by facet planes incrystal growth interfaces, dislocations are concentrated in the bottomsof the pits. All accumulated dislocations are not necessarilyintegrated, but form high-dislocation regions expanding without clearboundaries. In regions in which dislocations are concentrated withoutclear boundaries, it is considered that regions locally havingnonuniform carrier concentration distributions are formed by thediffusion of impurities.

Even in GaN crystals, in which the concentrations of dislocations in thebottoms of the pits are suppressed, nonuniform carrier concentrationdistributions may occur on their surfaces. If epitaxial GaN layers werecaused to grow on such GaN crystal substrates, ragged morphologies wouldappear on their surfaces, whose roughness is not substantially differentfrom that of GaN substrates with dislocation-concentrated regions. Thissuggests that the roughness of the epitaxial surface is caused not by adislocation density distribution but by a local distribution of thecarrier concentration.

If the growth of facets were terminated by increasing the amount ofhydrogen in a carrier gas as in JP 2003-178984 A, or by changing crystalgrowth conditions in the course of crystal growth, it might beconsidered that a crystal growth interface becomes flat, resulting in auniform surface carrier concentration distribution. However, becausethere has never conventionally been a concept of substantially uniformlycontrolling the carrier concentration distribution on the top surface ofthe substrate, the polishing of a substrate surface removes even regionshaving a uniform carrier concentration distribution, resulting in thelikelihood that there are large variations in the carrier concentrationon the mirror-finished substrate surface. No investigation hasconventionally been conducted as to how thick a surface layer having auniform carrier concentration distribution should be. Accordingly, evenif a GaN substrate with a surface layer having a uniform carrierconcentration distribution were produced, the mirror-finishing treatmentwould likely remove almost the entire surface layer or make it too thin.It has thus been impossible to stably produce a low-dislocation GaNsubstrate having small variations of a carrier concentration on thesurface and providing devices formed thereon with no defects.

OBJECTS OF THE INVENTION

Accordingly, an object of the present invention is to provide aself-supported substrate made of a III-V nitride semiconductor having asufficiently thick surface layer with a low dislocation density andreduced variations in a carrier concentration.

Another object of the present invention is to provide a method forproducing such a self-supported substrate.

SUMMARY OF THE INVENTION

As a result of intense research in view of the above objects, theinventor has found that (a) not only a uniformly reduced dislocationdensity but also a high in-plane uniformity of carrier concentration areimportant in a III-V nitride semiconductor substrate, on whichlight-emitting devices having uniform characteristics are formed with agood yield; that (b) when the III-V nitride semiconductor substrate hasa surface layer (at least up to a depth of 10 μm) having a substantiallyuniform carrier concentration distribution, an epitaxial GaN layer grownon the substrate is provided with uniform surface morphology andproperties; and that (c) when the surface layer having a substantiallyuniform carrier concentration is thinner than 10 μm, an epitaxial GaNlayer grown on the surface layer is provided with a rough surfacemorphology and a nonuniform composition of compound semiconductorcrystal, which reflects the carrier concentration distribution of thesubstrate.

When facets are caused to appear in a growth interface in the initialstage of growing a III-V nitride semiconductor substrate to bend thepropagation directions of dislocations thereby reducing the number ofdislocations reaching the top surface of the substrate, and when thegrowth interface becomes flat in the course of the crystal growth, it ispossible to grow a substrate having a uniform surface carrierconcentration distribution without increasing a dislocation density.Though effective for flattening the growth interface is to increase apartial pressure of hydrogen in a carrier gas in the course of the vaporphase growth, the growth interface can be made flat without changinggrowth conditions in the course of the crystal growth process when thepartial pressures of hydrogen and GaCl are relatively high since theinitial stage of crystal growth. The growth interface may also becomeflat by adding impurities such as Mg, etc. for accelerating the growthof the III-V nitride semiconductor in lateral directions.

The present invention, which has been achieved based on the abovefindings, is to provide a III-V nitride semiconductor substrate havingfew dislocations and a uniform surface (in-plane) carrier concentrationdistribution, on which an epitaxial GaN layer with high crystallinityand uniformity can grow, and a method for producing such a III-V nitridesemiconductor substrate.

Thus, the self-supported III-V nitride semiconductor substrate of thepresent invention has a substantially uniform carrier concentrationdistribution at least on the outermost surface, namely the top surface,thereof.

The self-supported III-V nitride semiconductor substrate according tothe first embodiment of the present invention has a substantiallyuniform carrier concentration distribution in a surface layer existingfrom the top surface to a depth of at least 10 μm.

The self-supported III-V nitride semiconductor substrate according tothe second embodiment of the present invention comprises a first layerhaving a plurality of regions different in a carrier concentration fromtheir surroundings in a direction substantially perpendicular to asubstrate surface, and a second layer existing from the top surface to adepth of at least 10 μm, the second layer being substantially free fromthe regions with different carrier concentrations, thereby having asubstantially uniform carrier concentration distribution.

The self-supported III-V nitride semiconductor substrate according tothe third embodiment of the present invention does not have clearboundaries between high-brightness regions and low-brightness regions ina fluorescence photomicrograph of its surface layer existing from thetop surface to a depth of at least 10 μm.

The self-supported III-V nitride semiconductor substrate according tothe fourth embodiment of the present invention comprises a first layerhaving high-brightness regions and low-brightness regions with clearboundaries and a second layer composed of a high-brightness region fromthe top surface to a depth of at least 10 μm in a fluorescencephotomicrograph in its arbitrary cross section, the low-brightnessregions and the high-brightness regions being different in a carrierconcentration.

The self-supported III-V nitride semiconductor substrate according tothe fifth embodiment of the present invention comprises substantially noregions different in a carrier concentration from their surroundings ina surface layer existing from the top surface to a depth of at least 10μm.

When the substrate has a carrier concentration of 1×10¹⁷ cm⁻³ or more,variations in the carrier concentration in the outermost surface (or thesurface layer or the second layer) are preferably within ±25%. When thesubstrate has a carrier concentration of less than 1×10¹⁷ cm⁻³,variations in the carrier concentration are preferably within ±100% inthe outermost surface (or the surface layer or the second layer).Variations in the carrier concentration are preferably not larger on itstop surface than on its bottom surface. Herein, variations in thecarrier concentration can be expressed by (a) (maximum value−minimumvalue)/average value of the carrier concentration, (b) deviation fromthe average value of the carrier concentration, (c) the standarddeviation of the carrier concentration, etc. Variations in the carrierconcentration herein are those calculated by (a) (maximum value−minimumvalue)/average value, unless otherwise described.

The regions with different carrier concentrations are, for instance,substantially in a planar shape with a wedge-like cross section, or in ashape of a cone, a hexagonal pyramid or a dodecahedral pyramid. Theregions with different carrier concentrations preferably have themaximum width of 1 mm or less.

The top surface or top and bottom surfaces of the III-V nitridesemiconductor substrate of the present invention are preferablypolished.

The III-V nitride semiconductor substrate of the present inventionpreferably has a thickness of 200 μm to 1 mm.

The top surface of the III-V nitride semiconductor substrate of thepresent invention is preferably a (0001) group-III surface.

The III-V nitride semiconductor substrate of the present inventionpreferably has a dislocation density lower on a top surface than on abottom surface.

The III-V nitride semiconductor substrate of the present inventionpreferably comprises a layer composed of GaN or AlGaN. The III-V nitridesemiconductor crystal is preferably doped with an impurity.

In the III-V nitride semiconductor substrate of the present invention,at least part of a III-V nitride semiconductor crystal is preferablygrown by an HVPE method.

The production method of the III-V nitride semiconductor substrateaccording to the first embodiment of the present invention comprises thesteps of growing a III-V nitride semiconductor crystal while forming aplurality of projections on a crystal growth interface at the initial orintermediate stage of crystal growth; conducting the crystal growthuntil recesses between the projections are buried, so that the crystalgrowth interface becomes flat; and continuing the crystal growth to athickness of 10 μm or more while keeping the crystal growth interfaceflat.

The production method of the III-V nitride semiconductor substrateaccording to the second embodiment of the present invention comprisesthe steps of (a) forming a first layer having a nonuniform carrierconcentration distribution, by growing a III-V nitride semiconductorcrystal while forming a plurality of projections on a crystal growthinterface at the initial or intermediate stage of crystal growth, and byfurther conducting the crystal growth until recesses between theprojections are buried, so that the crystal growth interface becomesflat; and (b) forming a second layer having a substantially uniformcarrier concentration distribution to a thickness of 10 μm or more bycontinuing the crystal growth while keeping the crystal growth interfaceflat.

The production method of the III-V nitride semiconductor substrateaccording to the third embodiment of the present invention comprises thesteps of (a) forming a first layer having a nonuniform carrierconcentration distribution, by growing a III-V nitride semiconductorcrystal while forming a plurality of projections on a crystal growthinterface at the initial or intermediate stage of crystal growth, and byfurther conducting the crystal growth until recesses between theprojections are buried, so that the crystal growth interface becomesflat; (b) forming a second layer having a substantially uniform carrierconcentration distribution by continuing the crystal growth whilekeeping the crystal growth interface flat; and (c) polishing a topsurface of the substrate after the completion of the crystal growth,such that a remaining second layer has a thickness of 10 μm or more.

The production method of the III-V nitride semiconductor substrateaccording to the fourth embodiment of the present invention comprisesthe steps of forming a III-V nitride semiconductor layer on a topsurface of a different substrate by epitaxial growth, and thenseparating the III-V nitride semiconductor layer from the differentsubstrate, wherein crystal growth is conducted while forming a pluralityof projections on a crystal growth interface at the initial orintermediate stage of growing the III-V nitride semiconductor layer;wherein crystal growth is then conducted until recesses between theprojections are buried, so that the crystal growth interface becomesflat; and wherein crystal growth is further continued to a thickness of10 μm or more while keeping the crystal growth interface flat.

The production method of the III-V nitride semiconductor substrateaccording to the fifth embodiment of the present invention comprises thesteps of forming a III-V nitride semiconductor layer on a top surface ofa different substrate by epitaxial growth, and separating the III-Vnitride semiconductor layer from the different substrate, (a) wherein afirst layer having a nonuniform carrier concentration distribution isformed by conducting crystal growth while forming a plurality ofprojections on a crystal growth interface at the initial or intermediatestage of growing the III-V nitride semiconductor layer, and by furtherconducting crystal growth until recesses between the projections areburied, so that the crystal growth interface becomes flat; and (b)wherein a second layer having a substantially uniform carrierconcentration distribution is formed to a thickness of 10 μm or more bycontinuing the crystal growth while keeping the crystal growth interfaceflat.

The production method of the III-V nitride semiconductor substrateaccording to the sixth embodiment of the present invention comprises thesteps of forming a III-V nitride semiconductor layer on a top surface ofa different substrate by epitaxial growth, and then separating the III-Vnitride semiconductor layer from the different substrate, (a) wherein afirst layer having a nonuniform carrier concentration distribution isformed, by conducting crystal growth while forming a plurality ofprojections on a crystal growth interface at the initial or intermediatestage of growing the III-V nitride semiconductor layer, and by furtherconducting the crystal growth until recesses between the projections areburied, so that the crystal growth interface becomes flat; (b) wherein asecond layer having a substantially uniform carrier concentrationdistribution is formed by continuing the crystal growth while keepingthe crystal growth interface flat; and (c) wherein a top surface of thesubstrate is polished after the completion of the crystal growth, suchthat a remaining second layer has a thickness of 10 μm or more.

The production method of the III-V nitride semiconductor substrateaccording to the seventh embodiment of the present invention comprisesthe steps of (a) forming a first layer having a nonuniform carrierconcentration distribution, by growing a III-V nitride semiconductorcrystal while forming a plurality of projections on a crystal growthinterface at the initial or intermediate stage of crystal growth, and byfurther growing the crystal until recesses between the projections areburied, so that the crystal growth interface becomes flat; (b) forming asecond layer having a substantially uniform carrier concentrationdistribution by continuing the crystal growth while keeping the crystalgrowth interface flat; and (c) removing at least part of the first layergrown while forming a plurality of projections on a crystal growthinterface, after the completion of the crystal growth.

The production method of the III-V nitride semiconductor substrateaccording to the eighth embodiment of the present invention comprisesthe steps of forming a III-V nitride semiconductor layer on a topsurface of a different substrate by epitaxial growth, and thenseparating the III-V nitride semiconductor layer from the differentsubstrate, (a) wherein a first layer having a nonuniform carrierconcentration distribution is formed, by growing the III-V nitridesemiconductor crystal layer while forming a plurality of projections ona crystal growth interface at the initial or intermediate stage ofcrystal growth, and by further conducting the crystal growth untilrecesses between the projections are buried, so that the crystal growthinterface becomes flat; (b) wherein a second layer having asubstantially uniform carrier concentration distribution is formed bycontinuing the crystal growth while keeping the crystal growth interfaceflat; and (c) wherein at least part of the first layer, which is grownwhile forming a plurality of projections on a crystal growth interface,is removed after the completion of the crystal growth.

The production method of the III-V nitride semiconductor substrateaccording to the ninth embodiment of the present invention comprises thesteps of (a) forming a first layer having a nonuniform carrierconcentration distribution, by growing a III-V nitride semiconductorcrystal while forming a plurality of projections on a crystal growthinterface at the initial or intermediate stage of crystal growth, and byfurther conducting the crystal growth until recesses between theprojections are buried, so that the crystal growth interface becomesflat; (b) forming a thick second layer having a substantially uniformcarrier concentration distribution by continuing the crystal growth fora long period of time while keeping the crystal growth interface flat;and (c) cutting the second layer in a direction perpendicular to thecrystal growth after the completion of the crystal growth, therebyobtaining a crystal substrate.

The production method of the III-V nitride semiconductor substrateaccording to the tenth embodiment of the present invention comprises thesteps of forming a III-V nitride semiconductor layer on a top surface ofa different substrate by epitaxial growth, and then separating the III-Vnitride semiconductor layer from the different substrate, (a) wherein afirst layer having a nonuniform carrier concentration distribution isformed, by growing the III-V nitride semiconductor layer while forming aplurality of projections on a crystal growth interface at the initial orintermediate stage of crystal growth, and by further conducting crystalgrowth until recesses between the projections are buried, so that thecrystal growth interface becomes flat; (b) wherein a thick second layerhaving a substantially uniform carrier concentration distribution isformed by continuing the crystal growth for a long period of time whilekeeping the crystal growth interface flat; and (c) wherein the secondlayer is cut in a direction perpendicular to the crystal growth afterthe completion of the crystal growth, thereby obtaining a crystalsubstrate.

In the production methods according to the seventh and eighthembodiments, it is preferable that at least part of the first layer,which is grown while forming a plurality of projections on a crystalgrowth interface, is removed by polishing the bottom surface of thesubstrate, so that the thickness of the substrate does not become lessthan 200 μm. The surface of the substrate is preferably mirror-polishedso that the thickness of the substrate does not become less than 200 μm.All of the first layer may be removed after the completion of thecrystal growth.

In the production method of the III-V nitride semiconductor substrate ofthe present invention, recesses in the roughness formed on the crystalgrowth interface at the initial or intermediate stage of the crystalgrowth are preferably (1) in a V-shaped or inversed-trapezoidal shape ina cross section in parallel to the crystal growth direction, which issurrounded by facet planes, or (2) in a conical shape surrounded byfacet planes.

In the production method of the III-V nitride semiconductor substrate ofthe present invention, at least part of the crystal growth is carriedout by an HVPE method.

In the production method of the III-V nitride semiconductor substrate ofthe present invention, it is preferable that to bury the roughness ofthe crystal growth interface during the crystal growth, a hydrogenconcentration in a growth atmosphere gas or the partial pressure of agroup-III source is made higher than in the previous steps.

In the production method according to the ninth or tenth embodiments,both top and bottom surfaces of the substrate cut out from thethick-grown crystal (second layer) are polished.

In a lot composed of a plurality of III-V nitride semiconductorsubstrates according to the present invention, all of the substrates arethe above III-V nitride semiconductor substrates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing the structure of theself-supported GaN substrate of the present invention (Example 1);

FIG. 2 is a schematic view showing one example of the production processof the self-supported GaN substrate of the present invention (Examples 1and 2);

FIG. 3 is a fluorescent photomicrograph showing a cross section of theself-supported GaN substrate of the present invention (Example 1);

FIG. 4 is a graph showing a carrier concentration distribution on a topsurface of the self-supported GaN substrate of the present invention(Example 1);

FIG. 5 is a graph showing a carrier concentration distribution on abottom surface of the self-supported GaN substrate of the presentinvention (Example 1);

FIG. 6 is a fluorescent photomicrograph showing the cross section of aconventional self-supported GaN substrate (Comparative Example 1);

FIG. 7 is a schematic view showing one example of the production processof the self-supported GaN substrate of the present invention (Example3);

FIG. 8 is a schematic view showing another example of the productionprocess of the self-supported GaN substrate of the present invention(Example 4);

FIG. 9 is a schematic view showing a further example of the productionprocess of the self-supported GaN substrate of the present invention(Example 5);

FIG. 10 is a fluorescent photomicrograph showing a cross section of theself-supported GaN substrate of the present invention (Example 5); and

FIG. 11 is a schematic view showing a still further example of theproduction process of the self-supported GaN substrate of the presentinvention (Example 6).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The term “self-supported substrate” used herein means a substrate havingsuch strength that is not only enough to maintain its shape but alsosuitable for handling. To have such strength, the self-supportedsubstrate preferably has a thickness of 200 μm or more. To make thecleavage easy after the formation of devices, the self-supportedsubstrate preferably has a thickness of 1 mm or less. When theself-supported substrate is too thick, the cleavage is difficult,resulting in a rough cleavage surface. If the self-supported substratewith a rough cleavage surface were used to produce semiconductor lasers,etc., they would have deteriorated characteristics due to reflectionloss.

The III-V nitride semiconductors, to which the present invention isapplicable, can be represented by the general formula:In_(x)Ga_(y)Al_(1-x-y)N, wherein 0≦x≦1, 0≦y≦1, and 0≦x+y≦1. Amongothers, semiconductors such as GaN, AlGaN, etc. are particularlypreferable because they satisfy the demand of substrate materials, suchas strength, production stability, etc.

Though regions having different carrier concentrations are notdistinguishable on the crystal surface by the naked eye, they are easilydetected by photoluminescence of the crystal caused by the irradiationof ultraviolet rays. In a crystal grown in the c-axis direction of ahexagonal crystal system, pits surrounded by facet planes are in ahexagonal or dodecagonal shape when viewed from the c-axis direction.Accordingly, its growth hysteresis is a hexagonal or dodecagonal prism.However, because each pit is small in the initial stage of the crystalgrowth, the growth hysteresis is generally a hexagonal or dodecagonalpyramid expanding as nearing the top surface of the substrate. When theregions surrounded by the facets are not pits but stripes grown througha mask by ELO such as FIELO, their hysteresis is a wedge or plate shapeor a shape resembled thereto, which is an inversed triangle or trapezoidin a cross section perpendicular to the stripe direction.

Because the regions having different carrier concentrations have clearboundaries with their surroundings, a fluorescence microscope can easilydetect them by image contrast. Though the detectable depth variesdepending on the wavelength and strength of ultraviolet rays asexcitation light, whether a detected image is seen on a sample surfaceor not is easily determined from an image-focusing position. Thedetection of the regions having different carrier concentrations mayalso be carried out easily by usual scanning electron microscopy (SEM)and cathodeluminescence (CL).

Each region having different carrier concentration in the III-V nitridesemiconductor substrate of the present invention preferably has a widthof 1 mm or less, because devices such as laser diodes and light-emittingdiodes produced by using such substrates have chip sizes of 1 mm orless. When the regions having different carrier concentrations arethicker than 1 mm, chips formed on the entire surface of the substrateare highly likely to be included in the regions having different carrierconcentrations, resulting in drastic decrease in the yield of devices.Of course, even when the regions having different carrier concentrationsare thicker than 1 mm, it is preferable that the surface carrierconcentration is highly uniform to avoid the effectiveness of thepresent invention from being hindered.

The substrate of the present invention desirably has a (0001) plane ofthe group III element on the surface. Because the GaN crystal has strongpolarity, and because its group III surface is more stable than itsgroup V surface (nitrogen surface) chemically and thermally, theproduction of devices is easier on the group III surface than on thegroup V surface.

The substrate of the present invention having decreased dislocationspropagating to its surface during crystal growth and a uniform carrierconcentration is characterized by a dislocation density that is lower ona top surface than on a bottom surface. For instance, with respect tothe dislocation density measured by an etch-pit method, etc., the topsurface of the substrate is desirably ½ or less of the bottom surface.

The growth of the III-V nitride semiconductor substrate of the presentinvention is preferably carried out by a hydride vapor-phase epitaxy(HVPE) method, because the HVPE method is advantageous in a high crystalgrowth rate, suitable for the production of substrates.

Because the absolute value of the carrier concentration of the III-Vnitride semiconductor substrate should properly be controlled dependingon devices to be produced, it cannot be determined without taking thetypes of devices, etc. into consideration. Because variations in thecarrier concentration should be changeable according to the absolutevalue of the carrier concentration, it cannot also be determined withouttaking into consideration the absolute value of the carrierconcentration. For instance, when an Si-doped, n-type GaN substrate tobe produced has a carrier concentration of about 1×10¹⁷ cm⁻³, variationsin the carrier concentration on the top surface of the substrate arepreferably within ±25%. When the carrier concentration is about 5×10¹⁷cm⁻³, variations in the carrier concentration on the top surface of thesubstrate are preferably within ±15%. When the carrier concentration isabout 5×10¹⁸ cm⁻³, variations in the carrier concentration on the topsurface of the substrate are preferably within ±10%. Incidentally, whenan Si-doped, n-type GaN substrate to be produced has a carrierconcentration of less than 1×10¹⁷ cm⁻³, variations in the carrierconcentration on the top surface of the substrate are preferably within±100%. As described above, a preferable range of variations in thecarrier concentration depends on the carrier concentration of thesubstrate, because the lower the carrier concentration in the substrateis, the less influence is caused by its large variations.

The type of conductivity of the substrate of the present invention mayproperly be selected depending on devices to be produced thereon.Examples of the conductivity type of the substrate of the presentinvention include an n-type doped with Si, S, O, etc.; a p-type dopedwith Mg, Zn, etc.; and semi-insulating-type doped with Fe, Cr, etc. orwith an n-type dopant and a p-type dopant simultaneously.

The surface of the III-V nitride semiconductor substrate (for instance,GaN substrate) of the present invention is preferably mirror-polished.The surface of the as-grown epitaxial GaN layer generally has a lot oflarge-scale roughness such as hillocks, etc., and a lot of small-scaleroughness that is considered to be formed by step bunching. Such roughsurface causes not only nonuniformity in morphology, film thickness,composition, etc. when an epitaxial layer is grown on the top surface ofthe substrate, but also low light exposure accuracy in aphotolithography process in the production of devices. Accordingly, thesubstrate desirably has a flat mirror surface. To obtain a mirrorsurface by polishing, the crystal surface should be scraped off to adepth of several to several hundreds of microns. In the presentinvention, the surface layer having a substantially uniform carrierconcentration should be as thick as 10 μm or more even after polishing.Therefore, in a case where the top surface of the substrate is to bepolished, the surface layer having a uniform carrier concentrationshould be grown in the crystal growth process to such a thickness thattakes into account a margin of polishing.

It is also desirable that the bottom surface of the III-V nitridesemiconductor substrate of the present invention is polished flat. Theself-supported substrate of the III-V nitride semiconductor (GaN, etc.)is usually obtained by heteroepitaxial growth on a different basesubstrate and peeling. Accordingly, it is likely that the as-peeledsubstrate has a rough bottom surface like a frosted glass, with part ofthe base substrate stuck thereto. In addition, some self-supportedsubstrates are not flat because of the warping of the substratesthemselves. These factors cause a nonuniform temperature distribution inthe substrate when an epitaxial layer is grown on the substrate,resulting in decrease in the uniformity of the epitaxial layer and thedeterioration of reproducibility.

When it is said that surface layer exists from a mirror-polished surfaceto a depth of at least 10 μm, it means that the surface layer has adepth of at least 10 μm after mirror-polished. Accordingly, the depth ofthis surface layer before mirror-polishing should be at least 10 μm+athickness removed off by mirror-polishing. The words “carrierconcentration distribution is substantially uniform” do not mean thatthe carrier concentration distribution is completely constant throughoutthe substrate, but mean that unevenness in the carrier concentration issmall enough to form devices with constant characteristics on thesubstrate. Therefore, in the case of the Si-doped, n-type GaN substrate(carrier concentration: about 5×10¹⁷ cm⁻³), they mean that unevenness inthe carrier concentration is within ±15%.

In the production method of the III-V nitride semiconductor substrate ofthe present invention, both top and bottom surfaces of the substrate cutout from the thick-grown crystal are preferably polished. Because thecut surface of the crystal generally has such roughness as saw marks,etc., it is difficult to achieve good epitaxial growth without working.The cutting of the crystal can be carried out by a slicer with aperipheral cutting edge, a slicer with an internal cutting edge, a wiresaw, etc. Among them, the wire-saw is preferably used.

Though the present invention is applied to the self-supported substratesof the III-V nitride semiconductors (GaN, etc.), the concept of thepresent invention is also applicable to epitaxial GaN substrates(templates), from which base substrates are not removed.

The present invention will be described in detail referring to Examplesbelow without intention of limiting the present invention thereto.

EXAMPLE 1

An epitaxial GaN layer was grown on a sapphire substrate, and thesapphire substrate was then removed to produce and evaluate aself-supported GaN substrate 1 having a layer containing regions 2having different carrier concentrations and a layer having asubstantially uniform carrier concentration as shown in FIG. 1. Theproduction method of the self-supported GaN substrate in this Examplewill be explained below referring to FIG. 2.

First, an epitaxial GaN layer 12 a was grown on a sapphire substrate 11by an HVPE method. The HVPE method is a method, in which GaCl, a halideof a group-III element, is transported onto a surface of a heated basesubstrate, and mixed with NH3 on the base substrate, so that they reactwith each other to cause the vapor-phase growth of a GaN crystal on thebase substrate 11. The source gases are supplied with carrier gases suchas H₂ and N₂. The temperature in a region surrounding the base substrate11 was set at 1000° C. by an electric furnace. The resultant GaN crystalwas doped with Si by supplying SiH₂Cl₂ as a doping gas to the substratein the growing process of the GaN crystal.

The partial pressures of GaCl and NH₃ as sources were 5×10⁻³ atm and 0.3atm, respectively, in a space surrounding the substrate. A mixed gas of2% of H₂ and 98% of N₂ was used as a carrier gas. Under theseconditions, nuclei of the GaN crystal 12 a were generated on thesapphire substrate 11 in a three-dimensional island growth mode. Withfacet planes appeared on sidewalls of each crystal nucleus, crystalgrowth progressed [step (b)]. This was confirmed by microscopicobservation on the surface and cross section of a substrate taken out ofthe furnace every growing time.

As the growing time became longer, the top portions of the GaN crystals12 a became flat [step (c)], and the GaN crystals 12 a continued growingin lateral directions to coalesce with each other so that their topsurfaces became further flat. However, because the growth interface didnot become completely flat, crystal growth progressed in a state wherethe substrate had many pits 13 on the surface [step (d)]. Each pit 13was substantially in a circular shape having a diameter of about severalto several tens of microns when viewed from right above. Observed in afluorescence photomicrograph showing the cross section of a sample atthe step (d) were dark regions 14 extending from an interface with thesapphire substrate 11 to the pits 13 exiting on the GaN surface. It ispresumed that these regions 14 contained a small amount of the dopant,with a lower carrier concentration than the surroundings. In fact, thecomparison of the SIMS analysis results of the dark regions 14 withthose of the neighbor regions in a fluorescent photomicrograph revealedthat the concentration of Si was 3×10¹⁷ cm⁻³ in the dark regions 14,while it was 7×10¹⁷ cm⁻³, twice or more, in the neighbor regions.

After growing the GaN crystal 12 a to the state (d), the growth of theGaN crystal 12 a was continued with the carrier gas changed to a mixedgas of 10% H₂ and 90% N₂, without changing the flow rate of the sourcegas.

As a result, it was observed that the growth interface 12 c of the GaNcrystal 12 a tended to become flat [step (e)]. After the growthinterface of the GaN crystal 12 a became flat, a GaN crystal 12 b wasgrown to a thickness of 100 μm or more. The fluorescent microscopicobservation of a cross section of the region 12 b grown after flatteningof the growth interface revealed that no different-brightness regionswere newly generated. That is, it was confirmed that thedifferent-brightness regions 14 were terminated in the middle of the GaNcrystal 12 (growth interface 12 c) [step (f)], and did not reach theoutermost surface 12 d of the GaN crystal 12. FIG. 3 shows a fluorescentphotomicrograph taken on the GaN crystal 12. Though part of pits did notterminate but reached the top surface, most pits were terminated in themiddle of the GaN crystal 12. It was confirmed that brightness in arange up to a depth of at least 10 μm from the crystal surface wassubstantially uniform in the fluorescent photomicrograph.

The GaN crystal 12 having a total thickness of 250 μm was thus grown onthe sapphire substrate 11. The average growth rate of the GaN crystal 12was about 50 μm/h.

A hetero-epitaxial substrate thus formed, which had the sapphiresubstrate 11 and the epitaxial GaN layer 12 thereon, was taken out of areaction tube, and the sapphire substrate 11 was removed from thehetero-epitaxial substrate to obtain a self-supported GaN substrate 15.The removal of the sapphire substrate 11 was conducted by a so-calledlaser lift-off method comprising irradiating the hetero-epitaxialsubstrate with high-power ultraviolet laser beams having such awavelength that can transmit the sapphire substrate 11 but is absorbedby GaN, from the side of the sapphire substrate 11 to melt an interfacewith the GaN crystal 12, thereby separating the GaN crystal 12. Inaddition, the sapphire substrate 11 can be removed, for instance, bymechanical polishing, or etching with a strong alkali or acid agent.Further, physical etching with ion or electron beams or neutron beamsmay be carried out for the removal of the sapphire substrate 11.

After removing a top surface portion and a bottom surface portion bothin a thickness of 10 μm from the self-supported GaN substrate 15 thusobtained, the self-supported GaN substrate 15 was mirror-polished toimprove its flatness. The self-supported GaN substrate 15 had a finalthickness of 230 μm, with sufficient strength to withstand handling withtweezers. The fluorescent microscopic observation of a cross section ofthe self-supported GaN substrate 15 revealed that there weresubstantially no regions with different carrier concentrations in asurface portion (to a depth of at least 10 μm) of the self-supported GaNsubstrate 15.

The carrier concentration distributions on top and bottom surfaces ofthis self-supported GaN substrate 15 were measured by a van der Pauwmethod at an interval of 5 mm in a diameter direction of the substrate.The results are shown in FIGS. 4 and 5. It was confirmed from FIG. 4that the carrier concentration on the top surface of the self-supportedGaN substrate 15 was as sufficiently uniform as 6.9×10¹⁷ cm⁻³ to7.6×10¹⁷ cm⁻³. On the contrary, it was found from FIG. 5 that thecarrier concentration on the bottom surface of the self-supported GaNsubstrate 15 were extremely nonuniform, from 2.7×10¹⁷ cm⁻³ to 7.1×10¹⁷cm⁻³.

An epitaxial GaN film was grown to a thickness of 1 μm on thisself-supported GaN substrate 15 by an MOVPE method, and its surfacemorphology was examined. As a result, it was confirmed that the entiretop surface of the epitaxial GaN film was a uniform mirror surface.

COMPARATIVE EXAMPLE 1

A thick GaN crystal layer was grown on a sapphire substrate in the samemanner as in Example 1 except that the partial pressures of GaCl and NH₃as source gases was 5×10⁻³ atm and 0.3 atm, respectively, on thesubstrate, and that a carrier gas used was a mixed gas of 2% H₂ and 98%N₂. As a result, many pits remained unburied on the surface until GaNbecame as thick as 300 μm.

This substrate was taken out of the reaction tube, and the sapphiresubstrate was removed by the above laser lift-off method to obtain aself-supported GaN substrate. Both top and bottom surfaces of theself-supported GaN substrate were mirror-polished to depths of 30 μm and10 μm, respectively, to improve the flatness of the substrate. Bymirror-polishing, almost all pits remaining on the surface of thesubstrate disappeared. The final thickness of the self-supported GaNsubstrate was 260 μm.

The cross section of the self-supported GaN substrate was observed by afluorescence microscope. As shown in FIG. 6, many wedge-shaped regionsextending between a top surface and a bottom surface and havingdifferent brightness from the surroundings existed in the substrate.

The carrier concentration distributions on the top and bottom surfacesof this self-supported GaN substrate were measured by a van der Pauwmethod at an interval of 5 mm in a diameter direction of the substrate.As a result, it was found that the carrier concentration on the topsurface of the substrate was as largely uneven as 2.4×10¹⁷ cm⁻³ to7.7×10¹⁷ cm⁻³, not so different from a nonuniform carrier concentrationon the bottom surface (2.6×10¹⁷ cm⁻³ to 8.1×10¹⁷ cm⁻³).

A 1-μm-thick epitaxial GaN film was grown on this self-supported GaNsubstrate by the MOVPE method. The observation of its surface morphologyconfirmed that many terrace-shaped projections of about 10 to 60 μm indiameter were formed on the entire surface of the epitaxial GaN film.These projections appear to be obstacles in the practical production ofdevices.

EXAMPLE 2

An epitaxial GaN layer was grown on a sapphire substrate substantiallyin the same manner as in Example 1 except for slight changes in crystalgrowth conditions in the HVPE method, and the sapphire substrate wasremoved to produce and evaluate a self-supported GaN substrate shown inFIG. 1. The production method of the self-supported GaN substrate inthis Example will be explained below referring to FIG. 2.

First, an epitaxial GaN layer 12 a was grown by the same HVPE method asin Example 1 using a C-plane sapphire substrate 11. The temperature onthe substrate was set at 1050° C. by an electric furnace. The partialpressures of GaCl and NH₃ as source gases were 6×10⁻³ atm and 0.4 atm,respectively, on the substrate, and the carrier gas used was a mixed gasof 10% H₂ and 90% N₂ from the initial stage. The GaN crystal was dopedwith Si by supplying SiH₂Cl₂ as a doping gas onto the substrate in theprocess of growing the GaN crystal.

First, nuclei of the GaN crystal 12 a were formed on the sapphiresubstrate 11 in a three-dimensional island mode, and facet planes thenappeared on the sidewalls of the crystal nuclei 12 a, progressingcrystal growth [step (b)]. This was confirmed by the microscopicobservation of the surface and cross section of the substrate taken outof a furnace after different growth time periods. As the growing timewent, the top portion of the GaN crystal 12 a became flat with a (0001)Ga plane above, and crystals growing in lateral directions thencoalesced with each other, resulting in further flattening of thesurface [step (c)]. As a result of further crystal growth under the sameconditions, pits 13 on the growth interface 12 c of the GaN crystal 12 awere spontaneously terminated, so that the substrate 12 a tended tobecome flat [step (e)]. Thus, after the growth interface 12 c of the GaNcrystal 12 a became flat, a GaN crystal 12 b was grown to a thickness of100 μm or more.

In the layer 12 b grown after the growth interface spontaneously becameflat, it was confirmed from the fluorescent microscopic observation ofits cross section that no different-brightness regions were newlygenerated. Thus, the regions 14 having different brightness endedhalfway without reaching the outermost surface 12 d of the GaN crystal12 [step (f)].

Thus, the GaN crystal 12 was grown in a total thickness of 550 μm on thesapphire substrate 11. The average growth speed of the GaN crystal 12was about 65 μm/h.

This substrate was taken out of the reaction tube, and the sapphiresubstrate 11 was removed by the above-described laser lift-off method toobtain a self-supported GaN substrate 15. Both top and bottom surfacesof the self-supported GaN substrate 15 was mirror-polished to remove atop surface portion of 30 μm and a bottom surface of 90 μm,respectively, to improve its flatness. The final thickness of theself-supported GaN substrate was 430 μm by mirror-polishing.

The carrier concentration distribution of this self-supported GaNsubstrate 15 on top and bottom surfaces was measured at an interval of 5mm in a diameter direction of the substrate by the van der Pauw method.As a result, it was confirmed that the carrier concentration on the topsurface of the substrate was sufficiently uniform in a range of 0.9×10¹⁸cm⁻³ to 1.6×10¹⁸ cm⁻³. On the other hand, the carrier concentration onthe bottom surface of the substrate was largely varying from 4.7×10¹⁷cm⁻³ to 13.1×10¹⁷ cm⁻³.

The fluorescent microscopic observation of the cross section of theresultant self-supported GaN substrate 15 revealed that there were nodifferent-brightness regions up to a depth of 100 μm or more from thetop surface of the substrate.

EXAMPLE 3

An epitaxial GaN layer was grown on a sapphire substrate by avoid-assisted separation (VAS) method, and the sapphire substrate wasthen removed to produce and evaluate a self-supported GaN substrate. Thedetails of the VAS method are described in JP 2003-178984 A. Briefly, itis a method in which crystal growth is carried out with a thin film oftitanium nitride having a network structure formed between the sapphiresubstrate and a growing GaN layer. A method for producing aself-supported GaN substrate in this Example will be described belowreferring to FIG. 7.

An undoped GaN layer 22 was grown to a thickness of 300 nm on a2-inch-diameter, C-plane substrate 21 of a single crystal sapphire, withtrimethylgallium (TMG) and NH₃ as source gases by an MOVPE method [step(b)]. A metal Ti film 23 was then vapor-deposited to a thickness of 20nm on this epitaxial GaN substrate 22 [step (c)], and the substrate wassubjected to a heat treatment at 1050° C. for 20 minutes in a stream ofa mixed gas of 20% NH₃ and 80% H₂ in an electric furnace. As a result,the GaN layer 22 was changed by partial etching to a layer 24 havingvoids generated at a high density, and the metal Ti film 23 was changedby nitriding to a TiN layer 25 having fine holes on the submicron ordergenerated at a high density. As a result, a substrate having a structureshown in (d) was obtained.

This substrate was placed in an HVPE furnace to deposit a GaN crystal 26to a total thickness of 400 μm. Source gases used for the growth of theGaN crystal 26 a were NH₃ and GaCl, and a carrier gas used was a mixedgas of 5% H₂ and 95% N₂. The growth conditions were normal pressure anda substrate temperature of 1040° C. The partial pressures of GaCl andNH₃ in the gas supplied were 8×10⁻³ atm and 5.6×10⁻² atm, respectively,to provide a V/III ratio of 7 at an initial stage of growth. Also,SiH₂Cl₂ as a doping gas was supplied onto the substrate, so that a GaNcrystal 26 a was doped with Si in its growth process.

Nuclei of GaN crystals 26 a were first grown on the substrate 21 in athree-dimensional island mode [step (e)], and the crystals growing inlateral directions then coalesed with each other so that their surfacesbecame flat [step (f)]. This was confirmed by the microscopicobservation of the surface and cross section of the substrate taken outof the furnace after different growth time periods. As the growth timepassed, the number of pits 27 in the growth interface of the GaN crystal26 a decreased, though crystal growth progressed in such a manner thatthe pits 27 did not completely disappear with many pits remaining on thesurface. Each pit 27 was in a circular or dodecagonal shape having adiameter of about several to several tens of microns when viewed fromright above. In a fluorescent photomicrograph of a cross section of asample corresponding to the step (f), there were dark regions 28extending from an interface with the substrate 21 to the bottoms of thepits 27 existing on the GaN surface. It is presumed that these regions28 contained a small amount of the dopant, with a lower carrierconcentration than the surroundings.

After GaN crystal 26 a was grown to a state shown in (f), crystal growthwas continued with the partial pressure of GaCl in the supplied gasincreased to 12×10⁻² atm. As a result, the pits 27 were terminated, andthe growth interface of the GaN crystal 26 a tended to become furtherflat [step (g)]. After the growth interface 26 c of the GaN crystal 26 abecame flat, the growth of the GaN crystal 26 b was further continued toa thickness of 200 μm or more. In a layer 26 b grown after theflattening of the growth interface, that fluorescent microscopicobservation of its cross section revealed that no different-brightnessregions were newly generated. It was thus confirmed that thedifferent-brightness regions 28 were terminated in the middle of the GaNcrystal 26 [step (h)], without reaching the outermost surface 26 d ofthe GaN crystal.

After the growth of the GaN crystal 26 was completed, the GaN layer 26spontaneously peeled off from the sapphire base substrate 21 with a voidlayer as a parting interface in the course of cooling the HVPEapparatus, to obtain a self-supported GaN substrate 30 [step (i)]. Thisself-supported GaN substrate 30 was mirror-polished on both top andbottom surfaces to remove a top surface portion to a depth of 20 μm anda bottom surface portion to a depth of 50 μm, thereby improving theflatness of the substrate 30. By mirror-polishing, the final thicknessof the self-supported GaN substrate 30 became 330 μm [step (j)].

The carrier concentration distribution of the resultant self-supportedGaN substrate 30 was measured by a van der Pauw method at an interval of5 mm in a diameter direction of the substrate on top and bottomsurfaces. As a result, it was confirmed that the carrier concentrationon the top surface 26 d of the substrate was sufficiently uniform in arange of 9.2×10¹⁷ to 10.1×10¹⁷ cm⁻³. On the contrary, the carrierconcentration on the bottom surface 26 e of the substrate was largelyvarying from 2.8×10¹⁷ cm⁻³ to 8.8×10¹⁷ cm⁻³. The fluorescent microscopicobservation of the cross section of this self-supported GaN substrate 30revealed that no different-brightness regions 28 existed up to a depthof 100 μm or more from the top surface 26 d.

The dislocation density of the self-supported GaN substrate 30 wasmeasured on their top and bottom surfaces 26 d, 26 e. The dislocationdensity on the top surface 26 d was determined by immersing theself-supported GaN substrate 30 in a heated mixture of phosphoric acidand sulfuric acid, and counting the number of pits generated by etching.The dislocation density on the bottom surface 26 e was determined from aplan-view TEM image. As a result, it was found that this self-supportedGaN substrate 30 had a dislocation density of 4.2±1×10⁶ cm⁻² on the topsurface 26 d, and a dislocation density of 7.2±1×10⁸ cm⁻² on the bottomsurface 26 e.

EXAMPLE 4

A self-supported GaN substrate was produced by growing an epitaxial GaNlayer on a sapphire substrate by a VAS method and then removing thesapphire substrate in the same manner as in Example 3, and evaluated.The production method of the self-supported GaN substrate in thisExample will be explained below referring to FIG. 8.

An undoped GaN layer 32 was grown to a thickness of 300 nm on a2-inch-diameter, C-plane substrate 31 of single crystal sapphire by anMOVPE method with TMG and NH₃ as source gases [step (b)]. A Ti metalfilm 33 was vapor-deposited to a thickness of 20 nm on this EpitaxialGaN substrate 32 [step (c)], and the substrate was subjected to a heattreatment at 1050° C. for 20 minutes in a stream of a mixed gas of 20%NH₃ and 80% H₂ in an electric furnace. As a result, the GaN layer 32 waschanged by partial etching to a layer 34 having voids generated at ahigh density, and the Ti layer 33 was changed by nitriding to a TiNlayer 35 having fine holes of submicrons at a high density, therebyproviding a substrate having a structure shown in (d).

This substrate was placed in an HVPE furnace, and a GaN crystal 36 wasgrown to a thickness of 550 μm. Source gases used for the crystal growthwere NH₃ and GaCl, and a carrier gas used was a mixed gas of 5% H₂ and95% N₂. The growth conditions were a normal pressure and a substratetemperature of 1040° C. The partial pressures of GaCl and NH₃ in the gassupplied were 8×10⁻³ atm and 5.6×10⁻² atm, respectively, to provide aV/III ratio of 7 at the time of starting crystal growth. Also, SiH₂Cl₂as a doping gas was supplied onto the substrate during the growth of theGaN crystal 36 a, so that the GaN crystal 36 a was doped with Si.

First, nuclei of GaN 36 a were generated in a three-dimensional islandmode on the substrate 31 [step (e)], and the crystals 36 a growing inlateral directions coalesced with each other, so that the GaN surfacesbecame flat [step (f)]. This was confirmed by the microscopicobservation of the surface and cross section of the substrate taken outof the furnace after different growth time periods. As the growth timepassed, the number of pits 37 on the growth interface of the GaN crystal36 a decreased. However, crystal growth progressed in a state in whichpits 37 did not completely disappear and many pits remained on thesurface. Each pit 37 was in a substantially circular or dodecagonalshape having a diameter of about several to several tens of microns whenviewed from right above. In a fluorescent photomicrograph of the crosssection of a sample corresponding to the step (f), dark regions 38extending from the substrate interface to the bottoms of the pits 37existing on the GaN surface were observed. It is presumed that theseregions 38 contained a small amount of the dopant, with a lower carrierconcentration than the surroundings.

After the GaN crystal 36 a was grown to a state shown in (f), crystalgrowth was continued with the partial pressure of GaCl in the suppliedgas increased to 12×10⁻² atm, so that the pits 37 were terminated, andthat the growth interface of the GaN crystal 36 a tended to become flat[step (g)]. Until this point, the GaN crystal 36 a was grown to athickness of approximately 80 μm. After the growth interface of the GaNcrystal 36 a became flat, the growth of GaN crystal 36 b was furthercontinued to a thickness of 470 μm. In the layer 36 b grown after thegrowth interface became flat, the fluorescent microscopic observation ofits cross section indicated that no different-brightness regions werenewly generated.

Namely, it was confirmed that the different-brightness regions 38 wereterminated in the middle of the GaN crystal 36 [step (h)], withoutreaching the outermost surface of the crystal.

During cooling the HVPE apparatus after the completion of crystalgrowth, the GaN layer 36 spontaneously peeled off from the basesubstrate 31 with a void layer as a parting interface, to obtain aself-supported GaN substrate 40, [step (i)]. This self-supported GaNsubstrate 40 was mirror-polished on both top and bottom surfaces 36 d,36 e, to remove top and bottom surface portions to depths of 20 μm and100 μm, respectively, thereby improving the flatness of the substrate 40[step (j)]. By mirror-polishing, the final thickness of theself-supported GaN substrate 40′ became 430 μm.

The carrier concentration distribution of the resultant self-supportedGaN substrate 40′ was measured on top and bottom surfaces 36 d, 36 f atan interval of 5 mm in a diameter direction of the substrate 40′ by avan der Pauw method. As a result, it was found that the carrierconcentration on the top surface 36 d of the substrate 40′ wassufficiently uniform in a range of 9.2×10¹⁷ cm⁻³ to 10.1×10¹⁷ cm⁻³. Itwas also found that the carrier concentration on the bottom surface 36 fof the substrate 40′ was 8.8×10¹⁷ cm⁻³ to 10.8×10¹⁷ cm⁻³, not largelydifferent from that on the top surface 36 d. The fluorescent microscopicobservation of the cross section of this self-supported GaN substrate40′ revealed that there were no different-brightness regions inside thesubstrate 40′.

EXAMPLE 5

An epitaxial GaN layer was grown on a sapphire substrate by the FIELOmethod (A. Usui, et al., Jpn. J. Appl. Phys. Vol. 36 (1997), pp.L.899-L.902), and the sapphire substrate was removed to obtain aself-supported GaN substrate for evaluation. The production method ofthe self-supported GaN substrate in this Example will be explained belowreferring to FIG. 9.

An undoped GaN layer 42 was grown to a thickness of 600 nm on a2-inch-diameter, C-plane substrate 41 of single crystal sapphire withTMG and NH₃ as source gases by an MOVPE method, [step (b)]. Next, anSiO₂ film was deposited to a thickness of 0.5 μm on this epitaxial GaNsubstrate by a thermal CVD method, and an SiO₂ film was provided withwindows in stripes in parallel to <11-20> by photolithography, to exposethe GaN layer 42 [step (c)]. The width of each window was 3 μm, and thewidth of an SiO₂ mask 43 was 7 μm.

This substrate was placed in an HVPE furnace, and a GaN crystal 44 wasgrown to a total thickness of 500 μm. Source gases used for the crystalgrowth were NH₃ and GaCl, and a carrier gas used was a mixed gas of 5%H₂ and 95% N₂. The growth conditions were normal pressure and asubstrate temperature of 1040° C. The partial pressures of GaCl and NH₃in the gas supplied were 8×10⁻³ atm and 5.6×10⁻² atm, respectively, atthe time of starting crystal growth, to provide a V/III ratio of 7.SiH₂Cl₂ as a doping gas was supplied onto the substrate in the course ofthe growth of the GaN crystal, so that the substrate was doped with Si.

A GaN crystal 44 was first grown selectively on the underlying GaN layerexposed to the windows in stripes in parallel to <11-20>. The crosssection of the GaN crystal 44 perpendicular to <11-20> was asschematically shown in (d).

When the windows of the mask were buried, a GaN crystal 44 a was grownin lateral directions on the SiO₂ mask 43 such that it covered theentire surface of the substrate. At this time, facet planes appeared onthe sidewalls of the GaN crystal 44 a extending in stripes, and grooves45 having a V-shaped cross section appeared in regions in which adjacentcrystals met [step (e)]. This was confirmed by the microscopicobservation of the surface and cross section of the substrate taken outof the furnace after different growth time periods.

In a fluorescent photomicrograph of a sample having a cross sectioncorresponding to the step (e), there were dark regions 46 from theinterface with the SiO₂ mask 43 to the bottoms of the V-shaped grooves45 existing on the GaN surface. These regions 46 contained a smallamount of a dopant, with a lower carrier concentration than thesurroundings.

As the crystal-growing time went, crystal growth progressed with theabove-described V-shaped grooves 45 remained on the growth interface.These grooves 45 were gradually buried as growth progressed, and whenthe thickness of the GaN crystal 44 a exceeded 100 μm, a GaN layerhaving a flat surface was obtained [step (f)].

After the growth interface of the GaN crystal 44 a became flat, thegrowth of GaN crystal 44 b was further continued to a thickness of about400 μm. The fluorescent microscopic observation of the cross section ofthe GaN crystal indicated that there were no new different-brightnessregions generated in a layer grown after the growth interface becameflat. Namely, it was observed that the different-brightness regions 46were terminated in the middle of the GaN crystal 44 [step (g)], withoutreaching the outermost surface 44 d of the crystal.

The GaN crystal 44 having a total thickness of about 500 μm was thusobtained. The average growth speed of the GaN crystal 44 was about 75μm/h. This substrate was taken out of the reaction tube, and thesapphire substrate 41 was removed by the above-described laser lift-offmethod, to obtain a self-supported GaN substrate 50 [step (h)].

The self-supported GaN substrate 50 was mirror-polished on both top andbottom surfaces 44 d, 44 e to remove top and bottom surface portions todepths of 20 μm and 60 μm, respectively, to improve its flatness [step(i)]. The final thickness of the self-supported GaN substrate 50 was 420μm. The fluorescent microscopic observation of the cross section of thesubstrate revealed that there were no regions with different carrierconcentrations in most of a top surface portion 44 b (up to a thicknessof 380 μm) of the substrate 50. FIG. 10 is a fluorescent photomicrographshowing the cross section of this self-supported GaN substrate 50.

The carrier concentration distribution of this self-supported GaNsubstrate 50 was measured on top and bottom surfaces 44 d, 44 e at aninterval of 5 mm in a diameter direction of the substrate by a van derPauw method. As a result, it was found that the carrier concentration onthe top surface 44 d of the substrate 50 was sufficiently uniform in arange of 6.6×10¹⁷ cm⁻³ to 7.2×10¹⁷ cm⁻³. On the contrary, it was foundthat the carrier concentration on the bottom surface 44 e of thesubstrate 50 was largely varying from 1.7×10¹⁷ cm⁻³ to 7.2×10¹⁷ cm⁻³.

An epitaxial GaN layer was grown to a thickness of 1 μm on thisself-supported GaN substrate 50 by an MOVPE method, and its surfacemorphology was examined. As a result, it was found that the entiresurface of the epitaxial GaN layer was in a uniform mirror state.

EXAMPLE 6

A first GaN layer 12 a containing regions 14 having different carrierconcentrations was grown on a sapphire substrate 11 having a diameter of50 mm by the same method and conditions as in Example 1 as shown in FIG.11 [steps (a) to (d)], and a growth interface 12 c was then caused tobecome flat [step (e)], so that a second GaN layer 12 b having a uniformcarrier concentration was grown [step (f)]. What is different fromExample 1 is that a second GaN layer 12 b having a uniform carrierconcentration was continuously grown to a thickness of about 20 mm.

The second GaN layer 12 b having a thickness of about 20 mm with thesapphire substrate 11 still attached thereto was mounted to a jig, andcut by a wire saw electrodeposited with abrasive diamond grains. Thecutting of the GaN crystal 12 b was conducted perpendicularly to thecrystal growth direction (in parallel to the surface of the sapphiresubstrate 11) [step (g)]. Thus, 19 GaN substrates 12 d of 50 mm indiameter and 450 μm in thickness were cut out from the secondthick-grown GaN layer 12 b. Each cutout GaN substrate wasmirror-polished on both top and bottom surfaces to obtain a colorless,transparent, self-supported GaN substrate 12 d [step (h)].

The fluorescent microscopic observation of arbitrary surface and crosssection of each self-supported GaN substrate 12 d thus obtainedindicated that it contained no different-brightness regions at all.

The carrier concentration distribution of each self-supported GaNsubstrate 12 d thus obtained was measured on a top surface in a diameterdirection of the substrate at an interval of 5 mm by the van der Pauwmethod. As a result, it was confirmed that the carrier concentration wassufficiently uniform in a range of 6.9×10¹⁷ cm⁻³ to 7.4×10¹⁷ cm⁻³.

An epitaxial GaN film was grown to a thickness of 2 μm on eachself-supported GaN substrate 12 d by an MOVPE method, and its surfacemorphology was observed. As a result, it was confirmed that it was in auniform mirror state on the entire surface of the epitaxial GaN film.

Though the present invention has been explained in detail abovereferring to Examples, it should be noted that the present invention isnot restricted thereto, and that various modifications such ascombinations of each process may be included within the scope of thepresent invention. For instance, in Examples, the MOVPE method may beused in part of the growth of the GaN crystal. In addition,conventionally known ELO technologies using SiO₂ masks, etc. may be usedin combination, to grow a crystal with a lot of roughness on the crystalgrowth interface in the initial or intermediate stage of crystal growth.Though the sapphire substrate was used as a base substrate in Examples,any substrates reported as conventional substrates such as GaAs, Si,ZrB₂, ZnO, etc. for epitaxial GaN layers may be used.

After removing the base substrate, the carrier concentrationdistribution on the surface of the GaN substrate may be made uniform bya heat treatment. This utilizes a phenomenon that atoms (or molecules)on a crystal surface are reconstructed by mass transport by keeping aGaN crystal at as high temperature as about 1000° C. In this method,however, a surface layer to be modified has a limited depth, so thatsuch effects of homogenization as in the present invention cannot beobtained.

Though the production method of the self-supported GaN substrate isillustrated in Examples, the present invention is of course applicableto a self-supported AlGaN substrate.

According to the present invention, the self-supported III-V nitridesemiconductor substrate having a low dislocation density and asubstantially uniform carrier concentration on its surface can beobtained lo stably. The self-supported III-V nitride semiconductor ofthe present invention makes it possible to produce at a high yielddevices such as light-emitting devices, electronic devices, etc. asdesigned.

1. A self-supported III-V nitride semiconductor substrate having asubstantially uniform carrier concentration distribution at least on itsoutermost surface.
 2. A self-supported III-V nitride semiconductorsubstrate having a substantially uniform carrier concentrationdistribution in a surface layer existing from the top surface to a depthof at least 10 μm.
 3. A self-supported III-V nitride semiconductorsubstrate comprising a first layer having a plurality of regionsdifferent in a carrier concentration from their surroundings in adirection substantially perpendicular to a substrate surface, and asecond layer existing from the top surface to a depth of at least 10 μm,said second layer being substantially free from said regions withdifferent carrier concentrations, thereby having a substantially uniformcarrier concentration distribution
 4. A self-supported III-V nitridesemiconductor substrate, wherein there re not high-brightness regionsand low-brightness regions with clear boundaries in a fluorescencephotomicrograph of its surface layer existing from the top surface to adepth of at least 10 μm.
 5. A self-supported III-V nitride semiconductorsubstrate comprising a first layer having high-brightness regions andlow-brightness regions with clear boundaries and a second layer composedof a high-brightness region mom the top surface to a depth of at least10 μm in a fluorescence photomicrograph in its arbitrary cross section,said low-brightness regions and said high-brightness regions beingdifferent in a carrier concentration.
 6. A self-supported III-V nitridesemiconductor substrate comprising substantially no regions different ina carrier concentration from their surroundings in a surface layerexisting from the top surface to a depth of at least 10 μm.
 7. The III-Vnitride semiconductor substrate according to claim 1, wherein saidsubstrate has a carrier concentration of 1×10¹⁷ cm⁻³ or more, andwherein variations in the carrier concentration are within ±5% in saidoutermost surface.
 8. The III-V nitride semiconductor substrateaccording to claim 2, wherein said substrate has a carrier concentrationof 1×10¹⁷ cm⁻³ or more, and wherein variations in the carrierconcentration are within ±25% in said surface layer.
 9. The III-Vnitride semiconductor substrate according to claim 3, wherein saidsubstrate has a carrier concentration of 1×10¹⁷ cm⁻³ or more, andwherein variations in the carrier concentration are within ±25% in saidsecond layer.
 10. The III-V nitride semiconductor substrate according toclaim 1, wherein said substrate has a carrier concentration of less than1×10¹⁷ cm⁻³, and wherein variations in the carrier concentration arewithin ±100% in said outermost surface.
 11. The III-V nitridesemiconductor substrate according to claim 2, wherein said substrate hasa carrier concentration of less than 1×10¹⁷ cm⁻³, and wherein variationsin the carrier concentration are within ±100% in said surface layer. 12.The III-V nitride semiconductor substrate according to claim 3, whereinsaid substrate has a carrier concentration of less than 1×10¹⁷ cm⁻³, andwherein variations in the carrier concentration are within ±100% in saidsecond layer.
 13. The III-V nitride semiconductor substrate according toclaim 1, wherein variations in the carrier concentration are not largeron its top surface than on its bottom surface.
 14. The III-V nitridesemiconductor substrate according to claim 3, wherein said regions withdifferent carrier concentrations are in a planar cape with a wedge-likecross section.
 15. The III-V nitride semiconductor substrate accordingto claim 3, wherein said regions with different carrier concentrationsare substantially in a shape of a cone, a hexagonal pyramid or adodecahedral pyramid.
 16. The III-V nitride semiconductor substrateaccording to claim 14, herein said regions with different carrierconcentrations have the maximum width of 1 mm or less.
 17. The III-Vnitride semiconductor substrate according to claim 1, wherein its topsurface is polished.
 18. The III-V nitride semiconductor substrateaccording to claim 1, wherein its bottom surface is polished.
 19. TheIII-V nitride semiconductor substrate according to claim 1, wherein ithas a thickness of 200 μm to 1 mm.
 20. The III-V nitride semiconductorsubstrate according to claim 1, wherein the top surface of saidsubstrate is a (0001) group-III surface.
 21. The III-V nitridesemiconductor substrate according to claim 1, wherein it has adislocation density lower on a top surface an on a bottom surface. 22.The III-V nitride semiconductor substrate according to claim 1, whereinit comprises a layer of GaN or Al GaN.
 23. The III-V nitridesemiconductor substrate according to claim 1, wherein said III-V nitridesemiconductor crystal is doped with an impurity.
 24. The III-V nitridesemiconductor substrate according to claim 1, wherein at least part ofsaid III-V nitride semiconductor crystal is grown by an HVPE method. 25.A method for producing a III-V nitride semiconductor substratecomprising growing a III-V nitride semiconductor crystal while forming aplurality of projections on a crystal growth interface at the initial orintermediate stage of crystal growth; conducting said crystal growthuntil, recesses between said projections are buried, so that saidcrystal growth interface becomes flat; and continuing said crystalgrowth to a thickness of 10 μm or more while keeping said crystal growthinterface flat.
 26. A method for producing a III-V nitride semiconductorsubstrate comprising (a) forming a first layer having a nonuniformcarrier concentration distribution, by growing a III-V nitridesemiconductor crystal while forming a plurality of projections on acrystal growth interface at the initial or intermediate stage of crystalgrowth, and by further conducting said crystal growth until recessesbetween said projections are buried, so that said crystal growthinterface becomes flat; and (b) forming a second layer having asubstantially uniform carrier concentration distribution to a thicknessof 10 μm or more by continuing said crystal growth while keeping saidcrystal growth interface flat.
 27. A method for producing a III-Vnitride semiconductor substrate comprising (a) forming a first layerhaving a nonuniform carrier concentration distribution, by growing aIII-V nitride semiconductor crystal while forming a plurality ofprojections on a crystal growth interface at the initial or intermediatestage of crystal growth, and by further conducting said crystal growthuntil recesses between said projections are buried, so that said crystalgrowth interface becomes flat; (b) forming a second layer having asubstantially uniform carrier concentration distribution by continuingsaid crystal growth while keeping said crystal growth interface it; and(c) polishing a top surface of said substrate after the completion ofsaid crystal growth, such that a remaining second layer has a thicknessof 1 μm or more.
 28. A method for producing a III-V nitridesemiconductor substrate comprising the steps of forming a III-V nitridesemiconductor layer on a top surface of a different substrate byepitaxial growth, and then separating said III-V nitride semiconductorlayer from said different substrate, wherein crystal growth is conductedwhile forming a plurality of rejections on a crystal growth interface atthe initial or intermediate stage of growing said III-V nitridesemiconductor layer; wherein crystal growth is then conducted untilrecesses between said projections are buried, so that said crystalgrowth interface becomes flat; and wherein crystal growth is furthercontinued to a thickness of 10 μm or more while keeping said crystalgrowth interface flat.
 29. A method for producing a III-V nitridesemiconductor substrate comprising the steps of forming a III-V nitridesemiconductor layer on a top surface of a different substrate byepitaxial growth, and separating said III-V nitride semiconductor layerfrom said different substrate, (a) wherein a first layer having anonuniform carrier concentration distribution is formed by conductingcrystal growth while forming a plurality of projections on a crystalgrowth interface at the initial or intermediate stage of growing saidIII-V nitride semiconductor layer, and by further conducting crystalgrowth until recesses between said projections are buried, so that saidcrystal growth interface becomes flat; and (b) wherein a second layerhaving a substantially uniform carrier concentration distribution isformed to a thickness of 10 μm or more by continuing said crystal growthwhile keeping said crystal growth interface flat.
 30. A method forproducing a III-V nitride semiconductor substrate comprising the stepsof forming a III-V nitride semiconductor layer on a top surface of adifferent substrate by epitaxial growth, and then separating said III-Vnitride semiconductor layer from said different substrate, (a) wherein afirst layer having a nonuniform carrier concentration distribution isformed, by conducting crystal growth while forming a plurality ofprojections on a crystal growth interface at the initial or intermediatestage of growing said III-V nitride semiconductor layer, and by furtherconducting said crystal growth until recesses between said projectionsare buried, so that said crystal growth interface becomes flat; (b)wherein a second layer having a substantially uniform carrierconcentration distribution is formed by continuing said crystal growthwhile keeping said crystal growth interface flat; and (c) wherein a topsurface of said substrate is polished after the completion of saidcrystal growth, such that a remaining second layer has a thickness of 10μm or more.
 31. A method for producing a III-V nitride semiconductorsubstrate comprising (a) forming a first layer having a nonuniformcarrier concentration distribution, by growing a III-V nitridesemiconductor crystal while forming a plurality of projections on acrystal growth interface at the initial or intermediate stage of crystalgrowth, and by further growing said crystal until recesses between saidprojections are buried, so that said crystal growth interface becomesflat; (b) forming a second layer having a substantially uniform carrierconcentration distribution by continuing said crystal growth whilekeeping said crystal growth interface flat; and (c) moving at least partof said first layer grown while forming a plurality of projections on acrystal growth interface, after the completion of said crystal growth.32. A method for producing a III-V nitride semiconductor substratecomprising the steps of forming a III-V nitride semiconductor layer on atop surface of a different substrate by epitaxial growth, and thenseparating said III-V nitride semiconductor layer from said differentsubstrate, (a) wherein a first layer having a nonuniform carrierconcentration distribution is formed, by growing said III-V nitridesemiconductor crystal layer while forming a plurality of projections ona crystal growth interface at the initial or intermediate stage ofcrystal growth, and by further conducting said crystal growth untilrecesses between said projections are buried, so that said crystalgrowth interface becomes flat; (b) wherein a second layer having asubstantially uniform carrier concentration distribution is formed ycontinuing said crystal growth while keeping said crystal growthinterface flat; and (c) wherein at least part of said first layer, whichis grown while forming a plurality of projections on a crystal growthinterface, is removed after the completion of said crystal growth. 33.The method for producing a III-V nitride semiconductor substrateaccording to claim 32, wherein at least part of said first layer, whichis grown while forming a plurality of projections on a crystal growthinterface, is removed by polishing the bottom surface of said substrate,so that the thickness of said substrate does not become less than 200μm.
 34. The method for producing a III-V nitride semiconductor substrateaccording to claim 32, wherein at least part of said first layer, whichis grown while forming a plurality of projections on a crystal growthinterface, is removed by polishing the bottom surface of said substrate,so that the thickness of said substrate does not become less than 200μm.
 35. A method for producing a III-V nitride semiconductor substratecomprising (a) forming a first layer having a nonuniform carrierconcentration distribution, by growing a III-V nitride semiconductorcrystal while forming a plurality of projections on a crystal growthinterface at the initial or intermediate stage of crystal growth, and byfurther conducting said crystal growth until recesses between saidprojections are buried, so that said crystal growth interface becomesflat; (b) forming a second layer having a substantially uniform carrierconcentration distribution by continuing said crystal growth whilekeeping said crystal growth interface at; and (c) cutting said secondlayer in a direction perpendicular to said crystal growth after thecompletion of said crystal growth, thereby obtaining a crystalsubstrate.
 36. A method for producing a III-V nitride semiconductorsubstrate comprising the steps of forming a III-V nitride semiconductorlayer on a top surface of a different substrate by epitaxial growth, andthen separating said III-V nitride semiconductor layer from saiddifferent substrate, (a) wherein a first layer having a nonuniformcarrier concentration distribution formed, by growing said III-V nitridesemiconductor layer while forming plurality of projections on a crystalgrowth interface at the initial or intermediate stage of crystal growth,and by further conducting crystal growth until recesses between saidprojections are buried, so that said crystal growth interface becomesflat; (b) wherein a second layer having a substantially uniform carrierconcentration distribution is formed by continuing said crystal growthwhile keeping said crystal growth interface at; and (c) wherein saidsecond layer is cut in a direction perpendicular to said crystal growthafter the completion of said crystal growth, thereby obtaining a crystalsubstrate.
 37. The method for producing a III-V nitride semiconductorsubstrate according to claim 31, wherein the top surface of saidsubstrate is mirror-polished so that the thickness of said substratedoes not become less than 200 μm.
 38. The method for producing a III-Vnitride semiconductor substrate according to claim 31, wherein all ofsaid first layer is removed.
 39. The method for producing III-V nitridesemiconductor substrate according to claim 25, wherein recesses in theroughness formed on said crystal growth interface at the initial orintermediate stage of said crystal growth are in a V-shaped orinversed-trapezoidal shape in a cross section in parallel to saidcrystal growth direction, which is surrounded by facet planes.
 40. Themethod for producing a III-V nitride semiconductor substrate accordingto claim 25, wherein recesses between projections formed in said crystalgrowth interface at the initial or intermediate stage of said crystalgrowth are in a conical shape surrounded by facet planes.
 41. The methodfor producing a III-V nitride semiconductor substrate according to claim25, wherein at least part of said crystal growth is carried out by anHVPE method.
 42. The method for producing a III-V nitride semiconductorsubstrate according to claim 25, wherein a hydrogen concentration in agrowth atmosphere gas is made higher than in the previous steps to burythe roughness of said crystal growth interface during said crystalgrowth.
 43. The method for producing a III-V nitride semiconductorsubstrate according to claim 25, wherein the partial pressure of agroup-III source is made higher than in the previous steps to bury theroughness of said crystal growth interface during said crystal growth.44. The method for producing a III-V nitride semiconductor substrateaccording to claim 35, wherein both top and bottom surfaces of a cutoutsubstrate are polished.
 45. A lot composed of a plurality of III-Vnitride semiconductor substrates, wherein all of said substrates are theIII-V nitride semiconductor substrates recited in claim 1.